Delayed thixotropic cement compositions and methods

ABSTRACT

Delayed thixotropic cement compositions and methods of cementing in subterranean zones penetrated by well bores are provided. The novel cement compositions are basically comprised of a hydraulic cement, sufficient water to form a slurry and a water soluble delayed thixotropy imparting additive selected from the group consisting of carbonic acid and alkali metal carbonates.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to thixotropic cement compositions and methods ofusing the same such as in cementing pipe strings in well bores withoutsubstantial loss of the cement composition.

2. Description of the Prior Art

Thixotropic cement compositions have been used heretofore for plugginglost circulation zones in well bores and for cementing pipe strings inwell bores containing fractures, vugs and other high permeability zones.The gel strengths of the thixotropic cement compositions are low duringmixing and pumping when the compositions are subjected to high shearstress, but when the compositions are subjected to static conditions,i.e., low or no shear stress, they quickly develop high gel strengthsand high viscosities. This property prevents substantial loss of thethixotropic compositions when they are placed across and enter lostcirculation zones.

The thixotropic cement compositions which have been utilized heretoforeoften include solid particulate thixotropy imparting agents which mustbe mixed with dry particulate cement and other solid additives prior toforming the solid materials into a slurry by the addition of mixingwater thereto. In offshore operations, the particulate solid materialsare dry blended onshore and several blends are taken to the offshorerig, e.g., thixotropic blends and non-thixotropic blends, so that theappropriate blend may be selected and mixed with water just before thecementing job is performed. This procedure of having several dryparticulate solid cement blends on the site lends itself to confusionand possible mistake and also creates the requirement that the unuseddry particulate blend must be returned or disposed of.

In well cementing operations such as primary cementing whereby pipestrings are sealingly cemented in well bores, thixotropic well cementingcompositions and methods which prevent substantial loss of the cementcompositions into fractures, vugs and the like in the well bore areadvantageous. Since the cement composition is circulated downwardlythrough the pipe string to be cemented and upwardly into and through theannulus between the pipe string and the walls of the well bore, it wouldalso be advantageous to utilize a delayed thixotropic cement compositionwhich does not develop thixotropic properties, and as a result is lessviscous and more easily pumped, until it has been pumped a substantialdistance through the pipe string.

Thus, there are continuing needs for improved thixotropic cementcompositions whereby the thixotropy imparting agent can be dissolved inthe mixing water instead of being added to the dry particulate solidcement and whereby the onset of thixotropic properties after the cementcomposition has been prepared is delayed.

SUMMARY OF THE INVENTION

The present invention provides delayed thixotropic cement compositionsand methods which meet the needs described above and overcome thedeficiencies of the prior art. The delayed thixotropic well cementingcompositions of the present invention are basically comprised of ahydraulic cement, sufficient water to form a slurry and a water solubledelayed thixotropy imparting additive selected from the group consistingof carbonic acid and alkali metal carbonates.

The methods of this invention for cementing in a subterranean zonepenetrated by a well bore are basically comprised of the followingsteps. A delayed thixotropic cement composition comprised of a hydrauliccement, sufficient water to form a slurry and a water soluble delayedthixotropy imparting additive selected from the group consisting ofcarbonic acid and alkali metal carbonates is prepared. Thereafter, thecement composition is pumped into the zone to be cemented and allowed toset into a hard impermeable mass therein.

It is, therefore, a general object of the present invention to provideimproved delayed thixotropic cement compositions and methods.

Other and further objects, features and advantages of the presentinvention will be readily apparent to those skilled in the art upon areading of the description of preferred embodiments which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows shear stress as a function of shear rate in a down/up sweepof a 13.9 lbs/gal thixotropic cement formulation at 50° C.

FIG. 2 shows the gel strength development of a thixotropic cementformulation at 50° C. at a constant frequency of 0.5 Hz and a constantstress of 2 Pa.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention provides improved delayed thixotropic cementcompositions and methods of using the cement compositions for cementingsubterranean zones penetrated by well bores. While the compositions andmethods can be utilized advantageously in both onshore and offshorewells, they are particularly advantageous in cementing offshore wellssince the thixotropy imparting additive of this invention can bedissolved in the mixing water and does not have to be premixed with thedry particulate solid cement and other particulate solid additives.Also, as mentioned, once the cement composition is mixed, e.g.,on-the-fly, the onset of the thixotropic properties in the cementcomposition is delayed for an initial period of time.

The delayed thixotropic well cementing compositions of this inventionare basically comprised of a hydraulic cement, sufficient water to forma slurry and a water soluble delayed thixotropy imparting additiveselected from the group consisting of carbonic acid and alkali metalcarbonates. Examples of alkali metal carbonates which can be utilizedinclude, but are not limited to, sodium carbonate, potassium carbonate,ammonium carbonate and lithium carbonate. Of the various alkali metalcarbonates and acids which can be utilized, sodium carbonate ispresently preferred. Generally, the thixotropy imparting additive ispresent in the thixotropic well cementing composition of this inventionin an amount in the range of from about 0.1% to about 8% by weight ofthe hydraulic cement in the composition, more preferably in the range offrom about 0.5% to about 4% and most preferably about 3%.

The thixotropy imparting additive can be dry blended with the cementused, added to a cement slurry after the slurry is formed, or it can bepredissolved in the mixing water so that the mixing water cansubsequently be mixed with the dry particulate solid cement and otherparticulate solid additives used (if any) to form a thixotropic cementcomposition of this invention just prior to utilizing the cementcomposition in a well cementing operation. The thixotropy impartingadditive in the cement composition reacts with calcium released duringthe ensuing cement hydration process. The onset of thixotropicproperties in the cement composition is delayed until a sufficientquantity of calcium is released. The calcium reacts with the thixotropyimparting additive to produce finely divided, colloidal particles ofcalcium carbonate which induce thixotropic properties in the resultingcement composition. Typically, substantial thixotropic properties arenot imparted to the cement composition for a time period up to about 5minutes after the components of the thixotropic cement compositions aremixed.

A variety of hydraulic cements which produce calcium when hydrated canbe utilized in accordance with the present invention. Such hydrauliccements include Portland cements, alumina cements, blast furnace slagcements and Pozzolanic cements. Of these, Portland cements or theirequivalents are generally preferred. Portland cements of the typesdefined and described in API Specification For Materials And Testing ForWell Cements, API Specification 10, Fifth Edition, dated Jul. 1, 1990 ofthe American Petroleum Institute are particularly suitable. PreferredAPI Portland cements include classes A, B, C, G and H, with API ClassesG and H being more preferred and class G being the most preferred.

The water utilized to form the cement compositions of this invention canbe fresh water, unsaturated salt solutions and saturated salt solutionsincluding brines and seawater. The water is included in the cementcompositions in an amount sufficient to form a slurry of the hydrauliccement and any insoluble particulate solid additives utilized.Generally, the water is present in the cement compositions in an amountin the range of from about 35% to about 100% by weight of hydrauliccement therein, more preferably in an amount of from about 40% to about90%.

As will be understood by those skilled in the art, the delayedthixotropic well cementing compositions of this invention can include avariety of cement composition additives including, but not limited to,set and compressive strength accelerating additives, set retarders,viscosifiers and the like. Examples of set and compressive strengthaccelerating additives which can be used include, but are not limitedto, sodium chloride, calcium chloride, triethanolamine, sodium silicateand sodium aluminate. Examples of set retarders include, but are notlimited to, lignosulfonates, acrylic acid/2-acrylamido-2-methylpropanesulfonic acid copolymers and carboxymethylcellulose. Viscosifiers whichcan be utilized include, but are not limited to, hydroxyethylcellulose,carboxymethylcellulose, guar gum, hydroxypropylguar, xanthan gum,scleroglican acrylate derivative copolymers and terpolymers, bentonite,hectorite and sepeolite. Viscosifiers reduce segregation (free water orsettling) in the cement composition.

A preferred delayed thixotropic well cementing composition of thisinvention is comprised of API Class G Portland cement, water for forminga slurry present in an amount in the range of from about 70% to about75% by weight of the cement in the composition and sodium carbonatepresent in an amount in the range of from about 3% to about 4% by weightof the cement in the composition.

The methods of the present invention for cementing in a subterraneanzone penetrated by a well bore are basically comprised of the followingsteps. A delayed thixotropic cement composition is prepared comprising ahydraulic cement, sufficient water to form a slurry and a water solubledelayed thixotropy imparting additive selected from the group consistingof carbonic acid and alkali metal carbonates. Thereafter, the cementcomposition is pumped into the subterranean zone to be cemented and thecement composition is allowed to set into a hard impermeable masstherein.

A more specific preferred method of cementing in a subterranean zonepenetrated by a well bore comprises the following steps. A delayedthixotropic cement composition is prepared comprising Portland cement,water for forming a slurry in an amount in the range of from about 44%to about 75% by weight of the cement in the composition and a watersoluble delayed thixotropy imparting additive comprised of sodiumcarbonate present in an amount in the range of from about 1% to about 4%by weight of the cement in the composition. The cement composition ispumped into the zone to be cemented and allowed to set into a hardimpermeable mass therein.

A method of the present invention for cementing a pipe string in a wellbore without substantial loss of cement into fractures, vugs and otherhighly permeable subterranean zones is comprised of the following steps.A delayed thixotropic cement composition is prepared comprising Portlandcement, sufficient water to form a slurry and a water soluble delayedthixotropy imparting additive selected from the group consisting ofcarbonic acid and alkali metal carbonates. The cement composition ispumped into the annulus between the pipe string and the walls of thewell bore and then allowed to set into a hard impermeable sealing masstherein. As mentioned, the water soluble delayed thixotropy impartingadditive can first be dissolved in the water, and just prior to pumpingthe cement composition into the annulus, the water is added to thecement and any additives utilized.

In order to further illustrate the delayed thixotropic cementcompositions and methods, the following examples are given.

EXAMPLE 1

Various thixotropic cement compositions of this invention were preparedcomprised of API Class G Portland cement, fresh water, a delayedthixotropy imparting additive comprised of sodium carbonate, ahydroxyethylcellulose viscosifier, and a sodium chloride acceleratingagent or a carboxymethylcellulose set retarder. The cement compositionswere mixed in a Waring blender and conditioned in an atmosphericconsistometer to the test temperature. After conditioning for 20minutes, the rheology of the cement compositions was determined inaccordance with the procedure set forth in the above mentioned APISpecification 10. Thereafter, the gel strength of the cementcompositions was determined after 10 seconds and after 10 minutes. Athixotropy imparting agent of this invention typically has a 10 secondgel strength only slightly higher than the 3 rpm reading in thethickening time test. The 10 minute gel strength is typically higherthan 100 lbs/100 ft². The results of the tests as well as the componentsof the test cement compositions and their quantities are given in theTable below. In addition, the test cement compositions were tested forfree water and the compressive strengths of the test cement compositionswere determined. These test results are also given in the Table below.

TABLE Thixotropic Cement Compositions And Test Results Test SamplesCement Composition Components 1 2 3 4 5 Water, parts by wt. 72.6 72.670.5 70.52 71.2 Thixotropy Imparting Additive¹, parts by wt. 4 3 4 3 4Hydroxyethylcellulose², parts by wt. 0.2 0.2 0.2 0.2 0.2 SodiumChloride³, parts by wt. 1.5 1.5 — — — Carboxymethylcellulose, parts bywt. — — — — 0.75 Cement⁵, parts by wt. 100 100 100 100 100 Density,lb/gal 13.9 13.9 13.9 13.9 13.9 Thickening Time Temperature, ° F. 100100 122 122 158 30 Bc, hr:min 3:59 4:36 4:03 3:45 4:08 70 Bc, hr:min4:18 5:52 4:34 5:33 5:00 100 Bc, hr:min — 7:52 4:45 6:03 5:13 Rheology,room temp. 300-200-100 70-51-30 108-81-53 85-63-40 100-76-52 165-130-8260-30-6-3 22-16-9-8 40-29-18-18 31-23-14-13 37-26-19-18 59-40-22-20Rheology Temperature, ° F. 100 100 122 122 158 300-200-100 75-56-3455-41-23 78-57-35 75-52-30 166-120-72 60-30-6-3 25-17-10-9 15-19-4-326-19-12-11 20-14-7-5 52-34-20-18 Gel Strength Temperature, ° F. 100 100122 122 158 10 sec/10 min 11/300+ 4/120 12/300+ 8/250 24/70 CompressiveStrength⁶ Temperature, ° F. 100 100 122 122 158 at 50 psi, hrs:min 6:160:20 3:55 6:41 4:53 at 500 psi, hrs:min 38:52  19:01  23:33  17.28 9:42in 24 hrs, psi — 520 — — — in 48 hrs, psi — 873 — — — ¹Sodium carbonateacid. ²Viscosifier ³Set and strength accelerator ⁴Set retarder⁵Dyckerhoff “G” ⁶Using Ultrasonic Cement Analyzer

From the Table, it can be seen that at varying concentrations of sodiumcarbonate, the initial gel strength of the compositions at theconditioning temperature is slightly higher than the 3 rpm rheologyreading, while in most cases, the 10 minute gel strength is higher than100 and often more than 300. The rheology after conditioning at roomtemperature was moderate to low. Furthermore, the test results show thatthe thickening time can be accelerated or retarded by about 5 hours andthe compositions set to 50 psi in less than 8 hours.

EXAMPLE 2

A delayed thixotropic cement composition of this invention was preparedby combining API Class G Portland cement with fresh water to form acement slurry having a density of 13.9 pounds per gallon. A sodiumcarbonate thixotropy imparting additive was dissolved in the water priorto mixing it with the cement. The cement composition contained thecomponents and amounts set forth in the preceding Table for TestSample 1. After mixing, the cement composition was stirred and heated to50° C. After conditioning the cement composition for 20 minutes at 50°C., the applied shear stress was reduced from 75 Pa to 10 Pa andincreased again to 75 Pa. The shear rate-shear stress curve shown inFIG. 1 was recorded using a constant strain rheometer during the shearstress reduction and increase.

As can be seen from FIG. 1, the shear rate when the shear stress wasreduced was much higher than the shear rate when the shear stress wasincreased which indicates that the cement composition was buildingadditional gel strength after being subjected to a lower stress which istypical thixotropic behavior.

EXAMPLE 3

In another test using a portion of the thixotropic cement compositiondescribed in Example 2, the gel strength development of the compositionwas determined at a constant stress over time. A thixotropic cementcomposition identical to that described in Example 2 was prepared,placed in a rheometer and heated to 50° C. After conditioning thecomposition for 20 minutes at 50° C., the stirring was stopped and anoscillation experiment was started to determine the gel strengthdevelopment of the composition. A constant frequency of 0.5 Hz and aconstant strain of 2 Pa was applied and the development of viscosityover time was measured. A graph of the viscosity v. time was preparedwhich is shown in FIG. 2. From FIG. 2, it can be seen that the viscosityof the thixotropic cement composition was constant for about the first250 seconds, but thereafter the viscosity increased over time to a finalgel strength of 126 Pa.

Thus, the present invention is well adapted to carry out the objects andattain the ends and advantages mentioned as well as those which areinherent therein. While numerous changes may be made by those skilled inthe art, such changes are encompassed within the spirit of thisinvention as defined by the appended claims.

What is claimed is:
 1. A method of cementing in a subterranean zonepenetrated by a well bore comprising the steps of: (a) preparing adelayed thixotropic cement composition comprising a hydraulic cement,sufficient water to form a slurry and a water soluble delayed thixotropyimparting additive selected from the group consisting of carbonic acidand alkali metal carbonates; (b) pumping said cement composition intosaid zone; and (c) allowing said cement composition to set into a hardimpermeable mass therein.
 2. The method of claim 1 wherein saidthixotropy imparting additive in said cement composition is sodiumcarbonate.
 3. The method of claim 1 wherein said thixotropy impartingadditive in said cement composition is present in an amount in the rangeof from about 0.1% to about 8% by weight of hydraulic cement therein. 4.The method of claim 1 wherein said hydraulic cement in said cementcomposition is Portland cement.
 5. The method of claim 1 wherein saidhydraulic cement in said cement composition is API Class G Portlandcement.
 6. The method of claim 1 wherein said water in said cementcomposition is selected from the group consisting of fresh water,unsaturated salt solutions and saturated salt solutions.
 7. The methodof claim 1 wherein said water in said cement composition is present inan amount in the range of from about 35% to about 100% by weight ofhydraulic cement therein.
 8. A method of cementing a pipe string in awell bore without substantial loss of cement into fractures, vugs andother highly permeable subterranean zones comprising the steps of: (a)preparing a delayed thixotropic cement composition comprising Portlandcement, sufficient water to form a slurry and a water soluble delayedthixotropy imparting additive comprised of sodium carbonate present inan amount in the range of from about 0.5% to about 4% by weight ofcement in said composition; (b) pumping said cement composition into theannulus between said pipe string and the walls of said well bore; and(c) allowing said cement composition to set into a hard impermeable masstherein.
 9. The method of claim 8 wherein said water is selected fromthe group consisting of fresh water, unsaturated salt solutions andsaturated salt solutions, and is present in said cement composition inan amount in the range of from about 40% to about 90% by weight ofcement therein.
 10. The method of claim 8 wherein said water solubledelayed thixotropy imparting additive is first dissolved in said waterand just prior to pumping said cement composition in accordance withstep (b), said cement is mixed with said water.
 11. The method of claim8 wherein said cement is API Class G Portland cement.